In all physical systems, energy moves ‘downhill’, from where there is more to where there is less. Heat travels from the hot sun into cold space. A truffle’s scent drifts from areas of high concentration to lower concentration. Neither has to be actively transported. As long as there is an energetic slope, energy will move from the ‘source’ (at the top) to the ‘sink’ (at the bottom). What matters most is how steep the slope is between the two.
